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Show 11~ GEOGRAPIIICAL DISTRIBUTION AND (Ch. VII. after studying the Holothurire, Medusre, and other congeners of delicate and changeable forms, came to the conclusion that each kind has its place of residence determined by the tem-perature necessary to support 1. ts ext·s te nce. Thus. ' for ex-ample, they found the abode of Pyrosoma ~tlantlc~ to be confined to one particular region of the Atlantic ocean · Let us now inquire how the transportation of polyps from one part of the globe to another is effected. Many of them, as in the families Flustra and Sertularia, attach themselves to sea-weed, and are occasionally drifted along with it. Many fix themselves to the shells of gasteropods, and are thus borne along by them to short distances. Some polyps, like the seapens, swim freely about in the sea. But the most frequent mode of transportation, probably, consists in the buoyancy of their eggs, or certain small vesicles which are detached and are capable of becoming the foundation of a new colony. These gems, as they have been ca1led, may be swept along by a wave that breaks upon a coral-reef, and may then be borne by a current to a distance. That some zoophytes adhere to floating bodies is proved by their being found attached to the bottoms of ships, as in the case of testacea before alluded to. Geographical Distribution and Migrations of Insects. Before we conclude our sketch of the manner in which the habitable parts of the earth are shared out among particular assemblaooes of organic beings, we must offer a few remarks 5 . on insects, which, by their numbers, and the variety of their powers and instincts, exert a prodigious influence in the economy of animate nature. As a large portion of these minute creatures are strictly dependent for their subsistence on certain species of vegetables, the entomological provinces must coincide in a considerable degree with the botanical. All the insects, says Latreille, brought from the eastern parts "' Voy. aux Terres Australes, tome i. p. 492. Ch. VII.] MIGRATIONS OF INSECTS. 113 of Asia and China, whatever be their latitude and temperature, are distinct from those of Europe and of Africa. The insects of the United States, although often they approach very close to our own, are nevertheless specifically distinguishable by some characters. In South America, the equinoctial lands of New Grenada and Peru on the one side, and of Guiana on the other, contain for the most part distinct groups; the Andes forming the division, and interposing a narrow line of severe cold between climates otherwise very similar *. The insects of the United States, even those of the northern provinces as far as Canada, differ specifically from the European, while those of Greenland appear to be in a g1·eat measure identical with our own. Some insects are very local, while a few, on the contrary, are common to remote countries, be ... tween which the torrid zone and the ocean intervene. Thus our painted lady butterfly (Vanessa Cardui) reappears in New Holland and Japan with scarcely a varying streak f. The same species is said to be one of the few insects which are universa11y dispersed over the earth, being found in Europe, Asia, Africa, and America; and its wide range is the more interesting because it seems explained by its migratory instinct, seconded, no uoubt, by a capacity enjoyed by few species, of enduring a great diversity of temperature. A vast swarm of this species, forming a column fl'Om ten to fifteen feet broad, was, a few years since, observed in the Canton de Vaud; they traversed the country with great rapidity from north to south, all flying onwards in regular order, close together, and not turning from their course on the approach of other objects. Professor Bonelli, of 'rurin, observed, in March of the same year, a similar swarm of the same species, also directing their flight from north to south, in Piedmont, in such immense numbers, that at night the flowers were literally covered with them. They had been traced from *Geographic Generale des Insectes et des Arachuid~s. Mem. du Mus. d'Hist Nat. tome iii. t Kirby and Spence, vol, iv. p. 487. v~~ 1 |
